Despite improvements in public hygiene, salmonellosis continues to cost the world economy billions of dollars each year and remains to be the number one cause of reported foodborne diseases. The Salmonella infection involves complex and highly orchestrated interactions between the bacterium and host cells. Salmonella injects proteins into host cells via a bacterial type III secretion system. Our working hypothesis is that these bacterial proteins engage host proteins for actin polymerization as well as depolymerization, two processes that are required for Salmonella-induced actin cytoskeleton rearrangements and invasion of non-phagocytic cells by the bacterium. The goal of this project is to identify and characterize bacterial and host proteins that play a role(s) in modulating actin dynamics both in vitro and in vivo by using microbiological, biochemical and cellular approaches. This proposal focuses on the molecular mechanism of Salmonella-induced actin rearrangements involving SipA. We have shown that SipA binds actin and modulates actin dynamics by decreasing the critical concentration for actin polymerization and by inhibiting depolymerization of actin filaments. We also showed that SipA increases the bundling activity of T-plastin, which increases the stability of actin bundles. Preliminary results indicate that additional host proteins are present in the SipA-actin complex and SipA activities must be turned off by other bacterial or host factors. We propose to investigate how Salmonella-induced actin cytoskeleton rearrangements are initiated, maintained and subsequently reversed. We have developed assays and reagents necessary to examine the actin architecture and investigate roles of SipA and host proteins in modulating Salmonella-induced actin cytoskeleton rearrangements. Results from this study will help us understand how Salmonella intercepts normal cellular constituents to modulate host actin cytoskeleton. A better understanding of these processes will facilitate the development of new chemotherapeutic agents for the treatment and prevention of salmonellosis. These experiments will also provide new insights into basic host cellular functions, including cytoskeletal rearrangements and cell movement.